They are to study the roles of cyclic AMP and cyclic GMP systems in the physiopathogenic process of cardiovascular system. It is hoped to define certain defects or modifications in the cyclic nucleotide systems in rats that have developed atherosclerosis, hypertension and failing hearts. It is found that cyclic AMP content in both aorta and ventricle from rats fed an atherogenic diet (containing high vitamin D and cholesterol) are lower than those in rats fed a basal diet. These observations are in good agreement with the findings that the activities of phosphodiesterases, both the low and high K sub m types, in aorta and ventricle from the atherogenic diet-fed rats have high affinities (lower K sub m's) than those of the respective tissues from rats fed basal diet.